Syngas, or synthesis gas, is a fuel gas mixture consisting primarily of hydrogen, carbon monoxide, and very often some carbon dioxide. The name comes from its use as intermediates in manufacturing synthetic natural gas (SNG) and for producing ammonia, methanol, and synthetic petroleum for use as a fuel or lubricant via the Fischer-Tropsch process. Syngas production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal or biomass, and in some types of waste-to-energy gasification facilities. Historically hydrogen has been produced in catalytic reforming, as a by-product of the production of the high-octane aromatic compounds used in gasoline. Such catalytic reforming process includes fixed bed, cyclic and continuous regeneration reformers. As hydrogen use has become more widespread in refineries, hydrogen production has moved to an integral feature of most refineries. This has been made necessary by the increase in hydrotreating and hydrocracking, including the treatment of progressively heavier feedstocks. Today, where by-product hydrogen production has not been adequate, steam reforming of methane has become the dominant method for hydrogen production.
The steam methane reformer (SMR) furnace is at the heart of every syngas generation plant. In all cases, the SMR radiant section contains several hundred vertically oriented straight centrifugally cast tubes, commonly known as “reformer” or “catalyst tubes” in which reactant gases at 450-650° C. and 30-45 bar arrive through an inlet manifold, are distributed to the reformer tubes filled with catalyst, heated to 800-900° C., and then collected and delivered to the next stage of the process. The tube metal temperature will be typically in the range of 875-975° C. The best feedstocks for steam reforming are light, saturated, and low in sulfur; this includes natural gas, refinery gas, liquefied petroleum gas, and light naphtha. These feeds can be converted to hydrogen at high thermal efficiency.
Alternatively partial oxidation (POX) can be used, particularly where heavy oil is available at low cost. POX reacted hydrocarbon feed with oxygen at high temperatures to produce a mixture of hydrogen and carbon monoxide. Since the high temperature takes the place of a catalyst, POX is not limited to the light, clean feedstocks required for steam reforming. Catalytic partial oxidation, also known as autothermal reforming (ATR), reacts oxygen with a light feedstock, passing the resulting hot mixture over a reforming catalyst. Since a catalyst is used, temperatures can be lower than in noncatalytic partial oxidation, which reduces the oxygen demand. Feedstock composition requirements are similar to those for steam reforming: light hydrocarbons from refinery gas to naphtha may be used. The oxygen substitutes for much of the steam in preventing coking, so a lower steam/carbon ratio can be used. Since a large excess of steam is not required, ATR produces more CO and less hydrogen than SMR, therefore it is suited to processes where CO is desired, for example, as syngas for chemical feedstocks and for synthetic petroleum to be used as a fuel or lubricant via the Fischer-Tropsch process. Incorporating gas heated reforming (GHR) or advanced gas heated reforming (AGHR) technologies into ATR-base syngas generation process offers combined benefit of significantly higher carbon efficiency and lower capital cost. The GHR recycles high grade heat from the reformed gas directly back into the reforming process. This reduces the oxygen requirement and eliminates the need to generate large quantities of high pressure steam from the process train.
POX requires an oxygen plant, which increases syngas generation cost significantly. In most of the Fischer-Tropsch based gas conversion processes, low cost syngas generation is important to achieving competitive advantage. Many schemes have been considered to achieve this objective, and one of them is to incorporate gas-gas heat exchangers in syngas generation process. Conventional shell and tube type heat exchanger can take hot syngas stream at 450-650° C. from ATR and preheat feedstocks such as natural gas before it enters into ATR. Since the heat duty of the reformer is greatly reduced, syngas generation cost can be improved. However, in such syngas generation processes, environments are encountered that have high carbon activities and relatively low oxygen activities.
High temperature reactor materials, heat exchanger materials, and syngas process tubing and piping materials used in these processes can deteriorate in service by a very aggressive form of corrosion known as metal dusting, which also inevitably accompanies coking.
In this disclosure, refinery process is not limited to syngas generation process, but includes other hydrocarbon conversion processes dealing with gas phase hydrocarbon process streams, e.g. feedstocks, products and effluent streams, at high temperatures. Such hydrocarbon conversion processes in refinery include catalytic cracking, catalytic reforming, alkylation, dehydrogenation, hydrocracking, hydrotreating, isomerization, visbreaking and coking processes. Such hydrocarbon conversion processes also include a process suitable for converting a variety of abundant hydrocarbon resources, including coal and petcoke, into valuable liquid fuels, synthetic natural gas, chemicals and other market competitive alternatives to petroleum products. Tubular conduits, or tubes, are utilized for at least (i) conveying the hydrocarbon feed, steam, oxygen, and mixtures thereof through the furnace's convection and radiant sections, (ii) conveying product effluent away from the radiant section, (iii) conveying a decoking mixture for removing coke or decoking effluent away from the radiant section, (iv) transferring heat to the hydrocarbon feedstocks, hydrocarbon products, syngas inside the tube for the steam reforming or partial oxidation reactions, and (v) conveying syngas product streams between process equipment via transfer lines and pipes at high temperatures. Therefore, in this disclosure, the refinery process tubes include any tubular conduits used in multiple refinery processes for hydrocarbon conversion, especially syngas generation tubes dealing with gas phase hydrocarbon process streams at high temperatures.
Regardless of the refinery process for hydrocarbon conversion, the formation of coke deposits on the refinery process tubes dealing with gas phase hydrocarbon process streams at high temperatures is not desirable. Coke deposits can lead to increased pressure in the tubes due to the restriction of flow, and to higher tube wall temperatures due to the insulating effects of the coke deposits. Both higher pressure and higher temperature lead to premature failure of the tubes. Furthermore, it is often necessary to periodically remove the tube from service and remove the coke deposits by burning off the deposited coke by oxidation with air, steam, and mixtures thereof or another oxidant that is passed through the tube at a high temperature. This periodic burn-off can result in severe thermal cycling, which also reduces the life of the tubes.
During normal use, the internal surfaces of the refinery process tubes dealing with gas phase hydrocarbon process streams at high temperatures could be subjected to metal dusting, carburization, sulfidation, and other forms of high temperature corrosion as a result of the prolonged exposure to the stream of hydrocarbons including syngas, heavy crude oil, resid and other petroleum fractions.
Metal dusting is a deleterious form of high temperature corrosion experienced by Fe, Ni and Co-based alloys at temperatures in the range of 350-1050° C. in carbon-supersaturated (carbon activity >1) environments having relatively low (about 10-10 to about 10-20 atmospheres) oxygen partial pressures. This form of corrosion is characterized by the disintegration of bulk metal into powder or dust. Most alloys that are commercially available today degrade by this corrosion process. Very fine dust particles originated from corrosion process are known to catalyze coke formation on the metal surfaces. Carburization is a form of high temperature degradation, which occurs when carbon from the environment diffuses into the metal, usually forming carbides in the matrix and along grain boundaries at temperatures generally in excess of 1000° F. (538° C.). Carburized material suffers an increase in hardness and often a substantial reduction in toughness, becoming embrittled to the point of exhibiting internal creep damage due to the increased volume of the carbides. Sulfidation is another form of high temperature corrosion in the refinery process. Sulfur is generally present as an impurity in fuels or feedstocks, and reacts with oxygen to form SO2 and SO3. An atmosphere of this type is generally oxidizing. Oxidizing environments are usually much less corrosive than reducing environments; here sulfur is in the form of H2S. However, sulfidation in oxidizing environments as well as in reducing environments is frequently accelerated by other impurities, such as sodium, potassium, and chlorine, which may react among themselves and/or with sulfur during combustion to form salt vapors. These salt vapors may then deposit at lower temperatures on metal surfaces, resulting in accelerated sulfidation attack. As is the case with oxidation, alloying with chromium enhances resistance to sulfidation, almost in direct proportion to the chromium content when hydrogen is not present.
Conventional heat-transfer tubes suitable for refinery processes dealing with gas phase hydrocarbon process streams at high temperatures are typically formed from an alloy comprising chromium, iron, and nickel, as well as various other elements, usually in low concentration, e.g., ≤5.0 wt. %, to obtain desired performance. These tubes also can be made out of stainless steels such as ferritic stainless steels, austenitic stainless steels, martensitic stainless steels, precipitation-hardenable (PH) stainless steels, and duplex stainless steels for further enhanced corrosion resistance, creep strength and rupture ductility. The typical composition of stainless steels used in the conventional tubes is shown in Table 1.
TABLE 1Typical Composition of Stainless Steels Used in the ConventionalTubesStainless SteelsNameUNS No.Alloy Compositions (Weight %)Ferritic stainless430S43000Bal.Fe:16.0~18.0Cr, 0.12C, 1.0Mn, 1.0Si, 0.04P, 0.03Ssteels434S43400Bal.Fe:16.0~18.0Cr:0.75~1.25Mo, 0.12C, 1.0Mn, 1.0Si,0.04P, 0.03SAustenitic302S30200Bal.Fe:17.0~19.0Cr:8.0~10.0Ni, 0.15C, 2.0Mn, 1.0Si, 0.045P,stainless steels0.03S304S30400Bal.Fe:18.0~20.0Cr:8.0~10.5Ni, 0.08C, 2.0Mn, 1.0Si, 0.045P,0.03S 304LS30403Bal.Fe:18.0~20.0Cr:8.0~12.0Ni, 0.03C, 2.0Mn, 1.0Si, 0.045P,0.03S310S31000Bal.Fe:24.0~26.0Cr:19.0~22.0Ni, 0.25C, 2.0Mn, 1.5Si,0.045P, 0.03S316S31600Bal.Fe:16.0~18.0Cr:10.0~14.0Ni:2.0~3.0 Mo, 0.08C, 2.0Mn,1.0Si, 0.045P, 0.03S 316LS31603Bal.Fe:16.0~18.0Cr:10.0~14.0Ni:2.0~3.0 Mo, 0.03C, 2.0Mn,1.0Si, 0.045P, 0.03S321S32100Bal.Fe:17.0~19.0Cr:9.0~12.0Ni:0.4Ti, 0.08C, 2.0Mn, 1.0Si,0.045P, 0.03S347S34700Bal.Fe:17.0~19.0Cr:9.0~13.0Ni:0.8~1.1Nb, 0.08C, 2.0Mn,1.0Si, 0.045P, 0.03SMartensitic 440CS44004Bal.Fe:16.0~18.0Cr:0.75Mo, 0.95~1.20C, 1.0Mn, 1.0Si,stainless steels0.04P, 0.03SPrecipitation-A286CS66286Bal.Fe:13.5~16.0Cr:24.0~27.0Ni:1.0~1.5Mo:0.35Al:1.9~2.35Hardenable (PH)Ti:0.10.5V:0.001~0.01B, 0.08C, 2.0Mn, 1.0Si, 0.04P, 0.03Sstainless steelsDuplex stainless2205CS31803Bal.Fe:21.0~23.0Cr:4.5~6.5Ni:2.5~3.5Mo:0.08~0.2N, 0.03C,steels2.0Mn, 1.0Si, 0.03P, 0.02S
The five classes of stainless steels are categorized as ferritic stainless steels, austenitic stainless steels, martensitic stainless steels, precipitation-hardenable (PH) stainless steels, and duplex stainless steels. Four out of five classes based on the characteristic crystallographic structure/microstructure of the alloys in the family: ferritic, martensitic, austenitic, or duplex (a mixture of austenitic and ferritic). The fifth class, the PH stainless steels, is based on the type of heat treatment used, rather than the microstructure.
Ferritic stainless steels are so named because their body-centered-cubic (bcc) crystal structure is the same as that of iron at room temperature. These alloys are magnetic and cannot be hardened by heat treatment. In general, ferritic stainless steels do not have particularly high strength. Their poor toughness and susceptibility to sensitization limit their fabricability and their useable section size. Ferritic stainless steels contain between 11 and 30 wt. % Cr, with only small amounts of austenite-forming elements, such as carbon, nitrogen, and nickel. Their general use depends on their chromium content. Austenitic stainless steels constitute the largest stainless family in terms of alloys and usage. They possess excellent ductility, formability and toughness and can be substantially hardened by cold work. Although nickel is the primary element used to stabilize austenite, carbon and nitrogen are also used because they are readily soluble in the face-centered-cubic (fcc) structure. A typical 300-series stainless steels contain between 17 and 22 wt. % Cr. Corrosion resistance of 300-series stainless steels depends on alloy content. Molybdenum is added to S31600 to enhance corrosion resistance in chloride environments. High-chromium grades such as S31000 are used in oxidizing environments and high-temperature applications. To prevent inter-granular corrosion after elevated-temperature exposure, titanium or niobium is added to stabilize carbon in S32100 or S34700. Also, lower-carbon grades (AISI L or S designations) such as S30403 (type 304L), have been established to prevent intergranular corrosion. Martensitic stainless steels are similar to iron-carbon alloys that are austenitized, hardened by quenching, and then tempered for increased ductility and toughness. Wear resistance for martensitic stainless steels is very dependent on carbon content. For instance, S44004 (1.1 wt. % C) has excellent adhesive and abrasive wear resistance similar to tool steels, whereas S41000 (0.1 wt. % C) has relatively poor rear resistance. PH stainless steels are chromium-nickel grades that can be hardened by an aging treatment. For instance, S66286 is an austenitic PH stainless steel and various alloying elements such as Al, Ti and Nb are used to form intermetallic compounds after aging. Duplex stainless steels are chromium-nickel-molybdenum alloys that are balanced to contain a mixture of austenite and ferrite. Their duplex structure results in improved stress-corrosion cracking resistance, compared with the austenitic stainless steels, and improved toughness and ductility, compared with the ferritic stainless steels. The original alloy in this family was the predominantly ferritic, but the addition of nitrogen to duplex alloys such as S31803 increases the amount of austenite to nearly 50%. It also provides improved as-welded corrosion properties, chloride corrosion resistance, and toughness.
Conventional heat-transfer tubes that need to be resistant to corrosion and coking are manufactured from alloys having desirable properties at high temperature, such as high creep-strength and high rupture-strength. Since the tubes are exposed to a high temperature corrosion environment during hydrocarbon processes, the alloy is typically corrosion-resistant. And since the tubes are exposed to an oxidizing environment during decoking, the alloy is typically oxidation-resistant. Conventional heat-transfer tube alloys include austenitic Fe—Cr—Ni heat resistant steels based on a composition having 25 wt. % chromium and 35 wt. % nickel (referred to as a “25 Cr/35 Ni alloy”), or a composition having 35 wt. % chromium and 45 wt. % nickel (referred to as a “35 Cr/45 Ni alloy”). It is conventional to employ differing compositions of minor alloying elements in order to enhance high temperature strength and/or carburization resistance and other corrosion resistance. The typical composition of conventional heat-transfer tube alloys is shown in Table 2.
TABLE 2Typical Composition of Conventional Heat-transfer Tube AlloysName of MaterialsElements in weight % (Balance is Fe)HP45Nb (25Cr/35Ni alloy)23~27Cr, 33~38Ni, 1.2~1.8Si, 1.2~1.7Mn, 0.2 max. Mo, 0.4~0.5C, 0.6~1.6Nb,0.020 max. P, 0.020 max. SHP16Nb (25Cr/35Ni alloy)22.5~26Cr, 35.5~37Ni, 1.2~1.8Si, 1.2~1.6Mn, 0.2 max. Mo, 0.14~0.18C,0.7~1.4Nb, 0.020 max. P, 0.020 max. SHN10NiNb18~23Cr, 31~34Ni, 0.8~1.3Si, 1.2~1.6Mn, 0.2 max. Mo, 0.09~0.12C, 0.8~1.2Nb,0.020 max. P, 0.020 max. SHP 40 Mod (25Cr/35Ni alloy)23.5~26.5Cr, 34~37Ni, 1.5~2.0Si, 1.25 max. Mo, 0.37~0.45C, other elements (W,Nb)Pompey HP 40W24~27Cr, 33~37Ni, 1.5~2.0Si, 1.5 max. Mo, 0.37~0.50C, 3.8~5.0WPompey Manaurite XM23~28Cr, 33~38Ni, 1.0~2.0Si, 1.0~1.5Mo, 0.37~0.50C, other additions (Nb, Ti,Zr)Manaurite XTM34~37Cr, 43~48Ni, 1.0~2.0Si, 1.0~2.0Mo, 0.4~0.45C, other additions (Nb, Ti)Kubota KHR 45A (35Cr/45Ni30~35Cr, 40~46Ni, 2.0 max. Si, 2.0 max. Mn, 0.4~0.6C, other additions (Nb, Ti)alloy)
In conventional heat-transfer tube alloys and stainless steels, a surface oxide comprising Cr2O3 typically forms during refinery processes dealing with gas phase hydrocarbon process streams at high temperatures. This oxide is believed to protect iron and nickel sites from contact with the gas phase hydrocarbon process streams, thereby lessening the amount of undesirable coke formation. It is observed, however, that under more severe refinery process conditions, e.g., conditions typically utilized for syngas generation dealing with gas phase hydrocarbon process streams at high temperatures, the formation of this protective oxide layer is suppressed in favor of carbon-containing phases, e.g., Cr3C2, Cr7C3, and/or Cr23C6. Accordingly, discontinuities develop over time in the corrosion-resistant scale located on the inner surface of refinery process tubes, resulting in iron and nickel exposure to the gas phase hydrocarbon process streams, leading to an increase in the rate of coke formation.
In an attempt to overcome this difficulty, U.S. Pat. Application Pub. No. 2012/0097289 discloses increasing the tube's carburization resistance by employing an alloy containing 5 to 10 wt. % aluminum. The alloy is said to form an Al2O3 scale during pyrolysis mode. It is reported that an Al2O3 scale remains in a stable oxide even under conditions where chromium preferentially forms carbides rather than oxides. Since such carburization-resistant alloys have a lower creep-strength and lower rupture-strength than do conventional heat-transfer tube alloys that do not contain aluminum, the reference discloses a tube structure wherein a continuous inner member formed from the aluminum-containing alloy is bonded to the inner surface of a tubular outer member which comprises a higher-strength alloy. While such tubes suppress coke formation, their dual-layer construction is economically demanding.
It is conventional to lessen the amount of aluminum in the steam cracker alloy in order to increase strength and thereby obviate the need for an outer member. See, e.g., U.S. Pat. No. 8,431,230, which discloses an aluminum-containing steam cracker alloy comprising 2 to 4 wt. % aluminum.
It is also conventional to increase the tube's heat transfer efficiency in order to expose the hydrocarbon and steam mixture to higher temperature and shorter contact time during pyrolysis, resulting in better selectivity for light olefin production. For example, increasing the heat transfer by increasing the tube's surface area that is exposed to the hydrocarbon feed is described in U.S. Pat. Nos. 6,419,885 and 6,719,953. Other methods for increasing the tube's heat transfer efficiency include the application of a mixing element (sometimes referred to as a “bead” or “fin”) on the inner surface of the heat transfer tube. For example, U.S. Pat. No. 5,950,718 describes the use of a conventional 25Cr/35Ni tube that includes a helical mixing element that is applied to the tube inner surface by plasma powder welding or arc welding. It has been observed that the flow of hydrocarbon and steam mixture through a radiant tube during pyrolysis results in the formation of a boundary layer adjacent to the radiant tube's inner surface. The boundary layer comprises hydrocarbon. The mixing element disturbs the boundary layer, leading to increased mixing between the boundary layer and the core flow of hydrocarbon and steam mixture. It is conventional to lessen the pressure-drop of the hydrocarbon and steam mixture traversing radiant tubes which contain one or more mixing elements. For example, U.S. Pat. No. 7,799,963 describes a structure which provides a decreased pressure drop as a result of discontinuities in the mixing elements. Both the tube and the discontinuous mixing elements are formed from conventional steam cracker alloys such as 25 Cr/20 Ni, 25 Cr/35 Ni, 35Cr/45Ni, or Incolloy™.
Nevertheless, there remains a need for heat transfer tubes that suppress the formation of chromium-carbide phases while providing improved heat transfer through the incorporation of mixing elements. There still remains a need for refinery process tubes that suppress corrosion and coke formation during refinery processes dealing with gas phase hydrocarbon process streams at high temperatures while providing improved heat transfer through the incorporation of mixing elements.